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IBO is 5 grains per pound. if you maintain the 5gpp ratio between the limbs, you theoretically shouldnt lose anything. reality doesnt always play by the same rules.

manufacturers use the 30"/70lb parameters because it looks better in numbers......and numbers sell bows.

When the war is over soldiers come home, SAILORS dont. WE will always stand a vigilant watch to protect YOUR way of life.
It's not that I'm cranky, I just dont care what you think.
Google isnt all that difficult to use, i'm not gonna spoon feed you info that you're too lazy to look up.

My XLR8 lost 1 -2 fps going from 70# to 60# as the poundage went down, and I maintained 5gpp.

I have a thread here somewhere where I show the draw weight, arrow weight, and chrono reading.

Over on the Mathews forum it was explained to me that as the poundage is lowered, the dl gets longer, and the brace lowers, accounting for the low loss in speed.

I didn't do that part of the experiment, but I didn't notice the measurement changes by feel at the time.

After I found my comfortable DW, I tweaked it a little, and found another 4 fps anyway...

When you decrease poundage by backing the limb bolts out, you will increase draw length and also INCREASE brace height. They will both increase because the limb tips move back as you back the bolts out which also moves the string back. If your brace height decreased your draw length would shorten. I would think the increase in brace and draw would cancel out any speed you'd gain but alot of it has to do with cam timing and rotation. You can sometimes over or under rotate cams for increased speed.

To the OP, if you shoot the same arrow at 70 and 60 lbs, the 60 lb draw will be about 20 fps slower. I got the following from Backcountry Bowhunting:

For every inch of draw length you go down you will lose 10 FPS

For every 5# draw weight you go down you will lose 10 FPS

For every 3 grains of weight your arrow is over IBO ( 5 grains per pound) you will lose 1 FPS

For every 3 grains of weight you add to your string you will lose 1 FPS.

there is going to be some drop in efficiency,your moving the same parts on the bow with less draw weight so something gets lost there,however due to the increase in dl and brace you loose less of it.if dl and brace stayed constant like say going from 70lb max limbs to 60lb max limbs you should loose more fps due to the brace and dl staying the same.i think kevin strother told me 6-10 fps if i remember correctly.

My XLR8 lost 1 -2 fps going from 70# to 60# as the poundage went down, and I maintained 5gpp.

I have a thread here somewhere where I show the draw weight, arrow weight, and chrono reading.

Over on the Mathews forum it was explained to me that as the poundage is lowered, the dl gets longer, and the brace lowers, accounting for the low loss in speed.

I didn't do that part of the experiment, but I didn't notice the measurement changes by feel at the time.

After I found my comfortable DW, I tweaked it a little, and found another 4 fps anyway...

Your brace height will actually increase as you decrease poundage... just think about the geometry. Or, better yet, do you have a string stopper on your bow? If the string is flush with the stopper at 70 lbs as you back it down to 60 lbs it will pull away from the stopper (AKA, longer brace height)

Your brace height will actually increase as you decrease poundage... just think about the geometry. Or, better yet, do you have a string stopper on your bow? If the string is flush with the stopper at 70 lbs as you back it down to 60 lbs it will pull away from the stopper (AKA, longer brace height)

When you decrease poundage by backing the limb bolts out, you will increase draw length and also INCREASE brace height. They will both increase because the limb tips move back as you back the bolts out which also moves the string back. If your brace height decreased your draw length would shorten. I would think the increase in brace and draw would cancel out any speed you'd gain but alot of it has to do with cam timing and rotation. You can sometimes over or under rotate cams for increased speed.

To the OP, if you shoot the same arrow at 70 and 60 lbs, the 60 lb draw will be about 20 fps slower. I got the following from Backcountry Bowhunting:

For every inch of draw length you go down you will lose 10 FPS

For every 5# draw weight you go down you will lose 10 FPS

For every 3 grains of weight your arrow is over IBO ( 5 grains per pound) you will lose 1 FPS

For every 3 grains of weight you add to your string you will lose 1 FPS.

this numbers are much more in line with what i have seen in real life,,,

10 pounds of weight loss will not give the same speed as 70 even if you maintain 5gpi,, if this were true a 40# bow at 5gpi would be as fast as a 70# bow.... its till being lanched at lower poundage.. the small difference in brace height and draw length is not worth trying to figure out,,as far as speed is concerned

i have found the speed studs from g5 to actually work and when placed right gain up to 8 fps.. in most bows. if placed wrong they will rob speed so that weight added to the string does not always cost you speed,,